Posts Tagged ‘Glory’

Why is our satellite mission named “Glory”? I can give you an explanation based on this short movie, which I captured from a downward looking camera on the B-200 during a recent flight.

While descending for landing, we encountered a cloud deck, and I received a clue for what, in geek terms, we call a scattering angle of 180 degrees, or backscattering direction (in this case, the scattering angle is the “Sun – point on cloud – me” angle).

In other words, I noticed the aircraft shadow on the clouds.

A glory captured by the NASA B-200 downward looking camera on June
5th, 2010.

Sun beams entering a cloud droplet are internally reflected, sort of like balls in a circular pool table. Some of them exit the droplet after one of these reflections, to form the regular rainbow (or “cloudbow” in this case, barely visible on the left of the frames).

Others take a few more bounces. Those that exit the droplet in the same direction they came from form a small circular rainbow called a Glory, whose features are enhanced when observed in polarized light. The prism “color-splitting” effect, as for the primary rainbow, is due to the slightly different deflection angles that different colors take when crossing the interface between air and water (or, for all that matters, between any two media with different refractive indices).

It is good practice to always search for the Glory. An ideal chance occurs when you fly over clouds, but remember that only your ability to see the airplane shadow will tell you if you chose the right seat. In proximity of the shadow, the alignment between you and the Sun is appropriate to make sure that the Sun beams are returned to you straight back by the cloud droplets.

For those scared who are scared to fly, yet are avid hikers, pay attention if you find yourself between the Sun and a fog or cloud bank. The multicolor crown developing around the shadow of your head can make you a saint, and gave name to this glorious optical phenomenon. Hallelujah!

The NASA Beech B-200 King Air taking flight. It requires the work of many people to have those wheels leaving the tarmac and land with data worth our scientific analyses, and sometimes with data at all.

There’re two instruments on the Beech King Air B200. One is the HSRL, which stands for High Spectral Resolution LIDAR. The latter word is an acronym born out of the marriage between RADAR and LASER. Even Radiohead use it. The one mounted on the B200 is an advanced device that my colleagues at NASA Langley have been extensively deploying. It pulses a laser beam straight downwards, and records the time it takes for light to come straight back after it bounces off particles. Based on this information, the atmosphere below the airplane is profiled and the height of aerosols layers effectively located.

HSRL "curtain plots' from earlier in the campaign, revealing layers of aerosol of different origins in the Southern California area.

The Research Scanning Polarimeter (RSP) is somewhat weirder, as polarization is a kind of information that is greatly masked to humans’ vision capabilities. Ask bees. If you wear sunglasses though, you should know that they cut the glare because they filter out light waves oscillating in all directions other than that vibrating in a certain direction.

Light is a wave, but most instruments can only measure the amplitude (=intensity) of it. Instead, the RSP adds to it by recording at which angle the light wave vibrates. As you can expect, the change of this angle as a light wave scatters off a a particle is extremely sensitive to the size and shape of the particle.

The RSP scans a line of pixels along the flight track, in the range +/- 60 degrees from the downward direction. Each scan takes about 1 second: as the aircraft proceeds, a point under the aircraft will be seen in subsequent scans at a different angle. The image on top is created by stacking the subsequent scans in columns. Features at different altitudes exhibit different slant angles as an effect of aircraft motion: if you understand what I'm trying to explain, you should be able to tell what causes the dark region on the ground, under the cloud. To be able to collect the information we want, we "straighten up" the data so that each column shows instead the same point seen from different angles (center). Here we straightened up the cloud. Many instruments stop at this point, but the RSP has the capability to measure also polarization. Look at the cloud rainbow emerging in stark contrast when observed in polarized light! Also, the surface loses contrast which is great because we can easily subtract it form the total signal (bottom). What's more important than looks is that the spacing of the rainbow colors drastically depends on the size of the cloud droplet. Similar concepts apply to aerosols.

We hope we can exploit the synergy between these two instruments to nail the vertical distribution and nature of aerosols in the atmosphere. Our studies confirm the potential of integrating the information derived by the complementary capabilities of the two instruments, sort of “Tell me where you are, and I’ll tell you who you are”.

Why do we do this? These instruments are “the little brothers” of the CALIPSO LIDAR (already in space), and the polarimeter that will be launched onboard the Glory mission at the end of this year. These satellites will fly shoulder by shoulder (just a few seconds apart!), therefore observing at any instant the same scene. You can even have your name orbiting with us!